Huntington's Disease (HD), also known as Huntington's Chorea, is a progressive disorder of motor, cognitive and psychiatric disturbances. The mean age of onset for this disease is age 35-44 years, although in about 10% of cases, onset occurs prior to age 21, and the average lifespan post-diagnosis of the disease is 15-18 years. Prevalence is about 3 to 7 among 100,000 people of western European descent.
Huntington's Disease is an example of a trinucleotide repeat expansion disorders were first characterized in the early 1990s (see Di Prospero and Fischbeck (2005) Nature Reviews Genetics 6:756-765). These disorders involve the localized expansion of unstable repeats of sets of three nucleotides and can result in loss of function of the gene in which the expanded repeat resides, a gain of toxic function, or both. Trinucleotide repeats can be located in any part of the gene, including non-coding and coding gene regions. Repeats located within the coding regions typically involve either a repeated glutamine encoding triplet (CAG) or an alanine encoding triplet (CGA). Expanded repeat regions within non-coding sequences can lead to aberrant expression of the gene while expanded repeats within coding regions (also known as codon reiteration disorders) may cause mis-folding and protein aggregation. The exact cause of the pathophysiology associated with the aberrant proteins is often not known. Typically, in the wild-type genes that are subject to trinucleotide expansion, these regions contain a variable number of repeat sequences in the normal population, but in the afflicted populations, the number of repeats can increase from a doubling to a log order increase in the number of repeats. In HD, repeats are inserted within the N terminal coding region of the large cytosolic protein Huntingtin (Htt). Normal Htt alleles contain 15-20 CAG repeats (SEQ ID NO: 226), while alleles containing 35 or more repeats (SEQ ID NO: 227) can be considered potentially HD causing alleles and confer risk for developing the disease. Alleles containing 36-39 repeats (SEQ ID NO: 228) are considered incompletely penetrant, and those individuals harboring those alleles may or may not develop the disease (or may develop symptoms later in life) while alleles containing 40 repeats or more (SEQ ID NO: 259) are considered completely penetrant. In fact, no asymptomatic persons containing HD alleles with this many repeats have been reported. Those individuals with juvenile onset HD (<21 years of age) are often found to have 60 or more CAG repeats (SEQ ID NO: 229). In addition to an increase in CAG repeats, it has also been shown that HD can involve +1 and +2 frameshifts within the repeat sequences such that the region will encode a poly-serine polypeptide (encoded by AGC repeats in the case of a +1 frameshift) track rather than poly-glutamine (Davies and Rubinsztein (2006) Journal of Medical Genetics 43:893-896).
In HD, the mutant Htt allele is usually inherited from one parent as a dominant trait. Any child born of a HD patient has a 50% chance of developing the disease if the other parent was not afflicted with the disorder. In some cases, a parent may have an intermediate HD allele and be asymptomatic while, due to repeat expansion, the child manifests the disease. In addition, the HD allele can also display a phenomenon known as anticipation wherein increasing severity or decreasing age of onset is observed over several generations due to the unstable nature of the repeat region during spermatogenesis.
Furthermore, trinucleotide expansion in Htt leads to neuronal loss in the medium spiny gamma-aminobutyric acid (GABA) projection neurons in the striatum, with neuronal loss also occurring in the neocortex. Medium spiny neurons that contain enkephalin and that project to the external globus pallidum are more involved than neurons that contain substance P and project to the internal globus pallidum. Other brain areas greatly affected in people with Huntington's disease include the substantia nigra, cortical layers 3, 5, and 6, the CA1 region of the hippocampus, the angular gyms in the parietal lobe, Purkinje cells of the cerebellum, lateral tuberal nuclei of the hypothalamus, and the centromedialparafascicular complex of the thalamus (Walker (2007) Lancet 369:218-228).
The role of the normal Htt protein is poorly understood, but it may be involved in neurogenesis, apoptotic cell death, and vesicle trafficking. In addition, there is evidence that wild-type Htt stimulates the production of brain-derived neurotrophic factor (BDNF), a pro-survival factor for the striatal neurons. It has been shown that progression of HD correlates with a decrease in BDNF expression in mouse models of HD (Zuccato, et al. (2005) Pharmacological Research 52(2):133-139), and that delivery of either BDNF or glial cell line-derived neurotrophic factor (GDNF) via adeno-associated viral (AAV) vector-mediated gene delivery may protect straital neurons in mouse models of HD (Kells, et al. (2004) Molecular Therapy 9(5):682-688).
Treatment options for HD are currently very limited. Some potential methodologies designed to prevent the toxicities associated with protein aggregation that occurs through the extended poly-glutamine tract such as overexpression of chaperonins or induction of the heat shock response with the compound geldanamycin have shown a reduction in these toxicities in in vitro models. Other treatments target the role of apoptosis in the clinical manifestations of the disease. For example, slowing of disease symptoms has been shown via blockage of caspase activity in animal models in the offspring of a pairing of mice where one parent contained a HD allele and the other parent had a dominant negative allele for caspase 1. Additionally, cleavage of mutant HD Htt by caspase may play a role in the pathogenicity of the disease. Transgenic mice carrying caspase-6 resistant mutant Htt were found to maintain normal neuronal function and did not develop striatal neurodegeneration as compared to mice carrying a non-caspase resistant mutant Htt allele (see Graham, et al. (2006) Cell 125:1179-1191). Molecules which target members of the apoptotic pathway have also been shown to have a slowing affect on symptomology. For example, the compounds zVAD-fmk and minocycline, both of which inhibit caspase activity, have been shown to slow disease manifestation in mice. The drug remacemide has also been used in small HD human trials because the compound was thought to prevent the binding of the mutant Htt to the NDMA receptor to prevent the exertion of toxic effects on the nerve cell. However, no statistically significant improvements were observed in neuron function in these trials. In addition, the Huntington Study Group conducted a randomized, double-blind study using Co-enzyme Q. Although a trend towards slower disease progression among patients that were treated with coenzyme Q10 was observed, there was no significant change in the rate of decline of total functional capacity. (Di Prospero and Fischbeck, ibid). U.S. Patent Publication No. 2011/0082093 discloses specific zinc finger proteins targeted to Htt.
Thus, there remains a need for compositions and methods for the treatment and prevention of Huntington's Disease.